and Laboratory Methods. 1555 



It is given as a general rule that with strong mineral acids orthosilicates 

 gelatinize, metasilicates yield pulverulent silica, and slimy silica indicates the 

 presence of both. 



The Orthosilicates. — Sa/fs. — -The only great family of normal salts is the 

 chrysolite group. Sepiolite is probably an acid salt. 



Derivatives. — The general formula is m R^Si04 • nA, in which A is an added 

 group of atoms. That these derivatives may fairly be said to consist of an 

 orthosilicate nucleus and added groups, is evidenced by : 



{(.i) The groups A may be added by influence of water or of heat. 



{U) Heat will split the derivative into nucleus and A if latter is volatile. 



One derivative passes into another by simple exchange of elements in A, 

 without affecting the nucleus. 



{d') If A involves a metasilicate the jelly of Si02 becomes slimy. 



These derivatives cannot be regarded as ordinary double compounds of A 

 and orthosilicate, for the properties of A are very variable; e.g., humite, 

 SMgjSiO^ • MgF2 does not simply split, but on heating yields SiF^. Serpen- 

 tine yields its water only at red heat. 



Only a few proportions between m and 71 are possible. For instance, the 

 orthosilicate Mg2Si04 and A=MgF2, if w=l then n^\ or 2 only. 



/^\ /^\ 

 Mg< >Si< >MgorMg2Si04. 



\o/ \o/ 



O. /0-MgF. 

 Mg< >Si< orMg2Si04.MgF2. 



\0/ \0— MgF 

 F— Mg— O. .0— MgF. 



yS'i/ or MgoSi04 . 2MgF<,. 



F— Mg— 0/ ^O— MgF. 

 Isomeres may be expected, for the unsymmetrical character of the structure 

 formula of Mg2Si04 . MgFjbecomes symmetrical if doubled. 



There are known the following five series of derivatives of orthosilicates : 

 1 M Q'n A J A=metasilicate. Serpentine group. 



1. n Mg2biU4 . A I A=MgF2Mg(OH)2. Chondrodite group. 



2. ?i Ni2Si04 . A. A=H2 0. Garnierite group. 



3. ;; Cu2Si04 .A. A^H20. Chrysocolla group. 



4. H Zn2 Sio4 . A. A==H20. Calamine group. 



5. fi Mn2Si04 . A. A=MnS, MnCl2, etc. Helvite group. 



The Metasilicates.— iVdV/t/rrr/ Sa/fs. — The very stable group of pyroxenes 

 and amphiboles not only are neutral salts but form derivatives, for it is an 

 excellent solvent for different alumosilicates and ferrisilicates, the best known 

 being R"Al2Si06 and R'2Al2Si40i2 (or R'2Fe2Si40i 2)- 



And Salts. — The metasilicates differ from orthosilicates in the formation of 

 chain-like compounds. For instance, the structure formula of talc H2Mg3Si40j 2 

 may be written 



HO— Si— O— Mg— O— Si— O— Mg— O— Si— O— Mg— O— Si— OH 



II II II 11 







whereas rensselaerite, with two more atoms of water than talc, is intermediate 

 between talc and serpentine, and with still two more atoms would pass into the 

 orthosilicates. 



(Continued in December.) 



